The berry contains an active glycoprotein molecule, with some trailing carbohydrate chains, called miraculin. When the fleshy part of the fruit is eaten, this molecule binds to the tongue's taste buds, causing sour foods to taste sweet. While the exact cause for this change is unknown, one hypothesis is that the effect may be caused if miraculin works by distorting the shape of sweetness receptors "so that they become responsive to acids, instead of sugar and other sweet things". This effect lasts 15-30 minutes.
Miracle fruit is available as freeze dried granules or in tablets - this form has a longer shelf life than fresh fruit. Tablets are made from compressed freeze dried fruit which causes the texture to be clearly visible even in tablet form.
The effect of Miracle fruit is made possible by contact with the tongue, not through digestion. For this reason, tablets must be allowed to dissolve in the mouth. The most pronounced effect can be achieved by coating the entire tongue in a paste of Miracle fruit for up to 30 seconds.
I just bought some off Thinkgeek.